High detection sensitivities of quantum absorptions are important in many research fields of physics, chemistry and biology. In this paper we present our latest results on the ultrasensitive molecular overtone spectroscopy using the cavity-enhanced frequency modulation (FM) technique. The principle of this method makes use of a high-finesse external cavity to enhance the intrinsic resonance contrast, while an FM modulation approach provides short-noise limited signal recovery. Ideal matching of the FM sideband frequency to the cavity free-spectral-range makes the detection process insensitive to laser frequency noise relative noise relative to the cavity, while at the same time overcomes the cavity bandwidth limit.Working with a 1064-nm Nd:YAG laser, we have obtained sub-Doppler overtone resonances of HCCD, HCCH and CO2 molecules. A detection sensitivity of 5 X 10-13 of integrated absorption over 1-s averaging time has been achieved. The resultant high signal- to-noise ratio of the weak resonance produces excellent laser frequency stabilization.© (1998) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.